Solder paste consists of finely granulated solder powder suspended in a viscous flux medium. Paste can be deposited in virtually any required shape or thickness by a number of different dispensing methods.

"Solder paste printing is universally accepted as the practical way to put solder paste on the surface mount device pads on the printed circuit board (PCB)," says William Coleman, vice president of technology at Photo Stencil (Colorado Springs, CO). Printing was developed concurrently with surface mount technology in the early 1980s, and made the surface mount assembly process economically competitive with through-hole assembly processes.

"Surface mount technology has been around since the late 1960s for the ceramic hybrid industries, but components were manually assembled," explains Douglas Dixon, product manager for Multicore soldering products at Henkel Loctite Corp. (Rocky Hill, CT). "The move into surface mount assembly with solder paste was driven by cost, size and performance requirements. Surface mount technology has benefits over through-hole technology in all areas."

No other solder material is as well-suited to high-quality, high-volume surface mount electronics assembly as solder paste. In fact, Dixon says approximately 75 percent of all electronics assembly today is done with solder paste, with through-hole assembly applications, such as wave soldering, making up the majority of the rest. "Solder paste used in applications other than printing makes up less than 1 percent of the total market," adds Dixon.

How It Works

Paste printing is the process of printing solder paste onto a substrate using a printer and a stencil. If the lot size is less than 50 PCBs, it is usually more economical to use a dispensing system. However, when the lot size is greater than 100 PCBs, screen printing is a more economical process.

Solder paste printing offers numerous advantages to electronics assemblers. "The real benefits of printing solder paste are speed and consistency of manufacturing," says Dixon. "Printing solder gives a very consistent volume of metal for each component joint. The alternatives to printing are through-hole or wave soldering, dispensing and manual assembly.

"All of these are niche applications and are less reliable and slower than surface mount printing," notes Dixon. "Typically, solder paste dispensing is done when the application [has] multilevel or nonplanar surfaces. For example, multichip modules where a flip chip component is already assembled on the substrate before the passives are added. The assembled flip chip requires paste to be deposited in alternative methods to printing."

Production quantities, deposit consistency requirements, tooling costs and solder paste yield all play a role in identifying the process with the best return on investment. "For companies with fairly stable product designs and moderate-to-high production volumes, stencil printing solder paste is usually the best choice with regards to cost, quality and throughput," says Vivari. "As production volumes shrink or design changes become more frequent, dispensing and screening of solder paste become more attractive options," adds Vivari. "Dispensing tends to be slower and screening does not produce the same high quality deposit. But, the trade-off in tooling costs and solder paste yield for dispensing can offset throughput considerations for low-volume applications."

Solder paste is printed onto the surface of an assembly through a stencil. Components to be soldered are placed on top of the paste, and the assembly is heated until solder particles within the paste reflow and form a contiguous solder fillet.

The stencil is a metal foil with numerous apertures cut into it corresponding to where paste should go onto the PCB. The stencil contacts the PCB with the pads and apertures aligned. A bead of solder paste is spread across the stencil stretching across the full length of the aperture array.

A squeegee blade is used to draw the paste across the stencil. The paste rolls across the stencil, filling the apertures as it moves. The stencil is lifted from the PCB leaving the solder deposits on the PCB pads.

"At its most basic, solder paste printing takes the form of a crashing wave of material," says Vivari. "When the wave passes over an aperture, paste crashes through the aperture, under the forces of gravity, inertia and downward pressure from the blade, and fills it in an almost backwards fashion with paste hitting the far wall and washing backwards until the entire aperture is filled.

"In cases where the aperture does not gasket to a surface, the paste will pass through the aperture and continue to flow until it reaches a barrier or the blade moves past, whichever happens first. When the blade behind the wave passes over the aperture, it acts to level off the deposited solder paste."

Printing Parameters

A perforated surface and something with a flat face to push the paste around are all that is required for solder paste printing. However, numerous printers are available with automated features that monitor and improve the printing process. Solder paste inspection machines are also available to detect printing defects.

"Solder paste printing can take any form, from a hand-cut paper stencil and a razor blade in an R&D lab to a half million dollar automated system including fully automatic stencil printers and 100 percent 3D paste deposition inspection equipment," says Vivari. "Equipment choice should be driven by production requirements."

Paste printers require squeegee blades and stencils. Squeegee speeds can vary widely between applications, but typically range around 1 to 3 ips. "Squeegee speed and squeegee pressure or force are directly proportional," says David Suraski, marketing manager at AIM Inc. (Cranston, RI). "The faster the squeegee speed is, the higher the squeegee pressure or force generally must be to achieve a good clean wipe of the top side of the stencil."

"As squeegee speed and pressure increase, so does the amount of heat that is generated at the squeegee-stencil interface. This promotes greater paste shear, which can result in several problems, such as slumping, bridging, and bleeding of paste underneath the stencil.

"High-speed printing can also cause squeegee blades and stencils to wear more quickly, resulting in incorrect solder deposition or volume, insufficient pad coverage and drag-out," warns Suraski. "High-speed printing typically requires better board support and more frequent stencil cleaning cycles."

According to Suraski, critical parameters, such as print speed, print pressure, separation speed and distance, should be controlled tightly to maintain print quality. "Noncritical parameters can then be utilized to optimize print cycle times," Suraski points out. "This method of printer control provides for a more repeatable print process and will allow your solder paste to maintain a more stable rheology.

"Squeegee pressure is the downward pressure, measured in pounds or kilograms per square inch, exerted by the squeegee blade onto the stencil surface during the print cycle," explains Suraski. "The purpose of squeegee pressure is to provide the force necessary to push the solder paste across the width of the printable area in a controlled roll, filling in all stencil apertures while providing a clean top-side wipe of the stencil surface.

"A typical starting point for squeegee pressure is 0.7 to 1.5 pounds of pressure per linear inch of printable area. Ideally, the squeegee blades should be the length of the printable area, plus 0.5-inch overhang on each side."

Separation distance is an adjustable distance to which the PCB and stencil separate at a pre-specified and controlled rate called separation speed. "Separation distance should be set great enough to allow all deposited paste to clear the stencil apertures prior to the increase in separation speed, which occurs when the separation distance set point has been reached," notes Suraski. He recommends starting with a separation distance of 0.1 inch to ensure total paste separation from stencil apertures.

"This may be reduced to a lesser distance once production has begun to satisfy cycle time requirements," explains Suraski. "If separation distance is reduced too much, however, print definition and quality will suffer."

Separation speed is an adjustable velocity that, in conjunction with separation distance, works to control post-print separation of the PCB and stencil. "In general, the slower the separation speed, the more reliable and repeatable the paste deposition," says Suraski. "Too fast of a separation speed can result in dog-ears, peaking or wicking, clogged stencil apertures, and poor paste coverage."

According to Tim Jensen, product specialist at Indium Corp. of America (Utica, NY), stencil design is critical. For instance, he says the stencil must deliver the correct amount of solder paste to surface mount pads as well as through-holes.

"Calculating stencil designs for a PCB can be cumbersome," warns Jensen. For example, the amount of paste deposited by the stencil varies with the size and shape of the apertures. Apertures can be circular, rectangular or square.

Making Stencils

There are three different stencil manufacturing methods: chemical etching, electroforming, and laser cutting. Each has advantages, but some processes are not suitable for producing wafer bumping stencils. For example, chemical etching cannot support the fine geometries and tight tolerances required for wafer bumping stencils. Laser cutting is preferred, because it produces apertures with smooth sidewalls.

"Chem-etch will not give the performance required for wafer bumping applications," claims Photo Stencil’s Coleman. "My preference is electroformed over laser because of the superior paste transfer efficiency."

For companies with frequent revision changes and custom production runs, EFD’s Vivari says screens [as opposed to stencils] may be the most cost-effective printing technology due to pure cost considerations. "While screening solder paste is generally not as consistent as stenciling solder paste, the requirement for consistency and what defines ‘acceptable quality’ is different from product to product," explains Vivari. "For ‘mainstream’ solder paste printing with stencils, the list of advantages includes the ability to automate, speed, repeatability and reproducibility."

Wafer bumping stencils can be used to print solder paste directly onto the pads of the wafer. After reflow, solder bumps are formed on the wafer. "The wafer is then cut and diced into die," says Coleman. "The die can then be attached to a substrate using flip chip assembly." The stencil usually has 25,000 to 500,000 apertures. According to Coleman, a typical stencil design would contain 0.06-inch circular apertures on a 16-by-16-inch foil.

Drawbacks

Because solder paste is a suspension of solder powder in flux, it is continually reacting. At normal room temperature, unopened paste will last no longer than 3 months. To slow the reaction down, paste should be refrigerated. With refrigeration, an unopened jar of paste can last up to 6 months.

"The flux medium contributes to the paste’s rheological properties and is susceptible to changes due to ambient conditions," says Henkel Loctite’s Dixon. "Changes in humidity and temperature affect the rheological conditions of solder paste. If not controlled, it will cause variability in the volume and consistency of solder paste deposit."

Passive components in the early 1980s were largely made up of 1206s, 2512s and larger components. Today’s passive component mix is largely made up of 0402s and even 0201s. "This component size trend has pushed solder paste powder size to finer mesh requirements," explains Dixon. "The smaller apertures put a greater demand on the release properties of the paste.

"As solder paste has become the main assembly process, higher volume line requirements are forcing faster print speeds to keep up with production," adds Dixon. "Faster print speeds require a higher thixotropic index and lower viscosities in order to maintain consistent print volumes."

In addition, changes in legislation are driving paste to be formulated with a larger variety of alloys. Dixon says the reflow requirements of these new alloys are changing the rheological and chemical nature of solder paste.

"An intrinsic limitation to the printing process is the requirement for a planar substrate," adds Dixon. "The stencil needs to be flat against the PCB for consistent volume. Inconsistent volume on a PCB cannot be reworked. The PCB must be cleaned and reprinted."

"The single biggest disadvantage of solder paste printing today is the level of training required to troubleshoot the inevitable problems that crop up," says Vivari. "A skilled employee can keep even the most cantankerous equipment humming along, while an unskilled employee can take your whole surface mount line down while trying to figure out why the print looks bad.

"Production yields depend on two factors: how well the printing process has been developed and how well your product design and inspection requirements dovetail with good board and stencil design practices," adds Vivari "You can choose the best stencil technology, stencil printer, solder paste and employee, but if your product design is poor or the stencil design breaks convention to accommodate ‘visual joint inspection requirements,’ your yields will likely be poor."